I had a strange experience the other day. I had 2 old 1/4″ straight bits (one marked Craftsman brand, the other had no markings) that I inherited with the used Craftsman router, my first ever router, I bought at least 20 years ago. I had never used these specific bits, to the best of my recollection. It was a non-critical job I was doing (making another jig) so I decided to try out these unknown bits.
I was routing a thru slot in 1/2″ hardwood-faced plywood, routing only 1/8″ deep at a time, and not exerting any unreasonable side force, when the bit broke, just plain broke with no violence, at the shank end of the cutting section. I recovered the broken section from my table enclosure. I put the other bit (didn’t check whether the marked bit was first or last) and it, too, broke almost identically.
Examining the fractured surface, there are no obvious defects in the material. The grain of the metal just gave up, leaving a rough, grainy surface on both pieces.
Has anyone else ever seen such a thing happen under any circumstances? I’m baffled.
Replies
Take a good look at the collet and make sure it is clean, that it is not gripping unevenly at the outer end allowing the bit to wobble very slightly when tightened normally. Also consider using 1/2 " shanks as much as possible.Look for any local polished areas on the shank indicating movement
Thanks for the response, Jako. I do use 1/2" shanks whenever I can. And 8mm or 3/8" where appropriate, as well. Still, I have never broken a bit in quite this manner before.
I've burned bits, even had carbide chip off, but never a bit just break, in any diameter. And these broke below the shank anyway, down in the cutting area of the bit, but not as far down as the cutting was occurring. Maybe about half way between the shank and the cutting point. Of course, I was using them in a router table so down the bit is up and the shank end is down. Confusing enough?
In fact, these bits were chucked into a 1/4-1/2" adapter to fit my 1/2" collet, to save the time of switching the collets. I keep them impeccably clean. It's always worked just fine before. And there are no polished areas on the broken shanks.
Remember that I said that these bits were old and had never been used before. I think it likely that they were just poor quality since they broke on first use in a very undemanding situation. I normally use spiral upcut bits for mortising and slotting but I used these to see how well they might work in future routing, but they're no great loss. I'll just replace them with known industrial quality bits.
Truth is, indeed, stranger than fiction.--
Lee in Cave Junction, Oregon
Gateway to the Oregon Caves
I am not the best person to answer this question but in one of the Sawing Technology workshops I ran at OSU for sawfilers, Warren Bird of Cal Saw spoke extensively on the formation of martensite when tooling is improperly heated/cooled (as with brazing of carbide or stellite tipping processes).
Martensite is a hard and brittle iron compound and I remember Warren talking about it in relation to broken teeth on saws.
I, for a long time, felt that I should always use 0.5" shank router bits but I got a lot of broken bits especially those with 0.25" and 0.375" cutting diameters. The bits would break off just above the point of brazing where the tooling flared up to the 0.5" shaft. On one occasion, I put in a new bit -- ran it about 18" when it broke off. So I took the other new bit from the same manufacturer, chucked it up and ran it about 24" inches before it too broke off at basically the same spot. Besides being extremely pissed off, I came to the gestalt realization that the tooling was improperly made and that I should have paid more attention to Warren's presentation.
I always felt that the tooling was improperly annealed/tempered/whatever (somebody help me out on this) and as such martensite was formed in this transition area during the tippin process.
Once I got over my hangup about always opting for the largest shank/collet diameters and got some good quality 0.25" solid carbide 0.25" shaft bits, my breakage problem was eliminated.
Stanley, your comments on martensite ring true. The breaks were not quite in identical locations, but they were both real close to the location you mention and the grainy appearance of the fracture zone is consistant with your premise. Thank you.
--
Lee in Cave Junction, Oregon
Gateway to the Oregon Caves
I doubt if one can find the answer easily not without a metallurgist having a look and at risk of being accused of flogging a dead horse, one other thought came to mind.As mentioned by the other poster re martensite etc. Plunging a 2 flute straight cutter as against a spiral causes intense vibration in the bit .Collet adapters introduce more flex in the connection and he may have the answer.If one didn't have to earn a living woodworking and the safety aspect could be addressed it would be interesting to try it again with one of Charles's fine bits.Maybe he has some comments here. having had a serrated back shaper knife shatter some time ago and not been disemboweled the steel dust boot wasn't so lucky! I am a tad nervous of tooling breaking.
It was a plunge cut, Jako, but done in a router table lowering the workpiece onto the bit. The workpiece was only 1/4" ply and I felt no no noticable vibration. The break occurred after advancing some inches from the plunge entry point, not forcing the cut in any way.
These are high quality adapters from Lee Valley, well and accurately machined.
Glad you weren't hurt when your shaper bit blew up. I, too, worry about such things, but I try not to let it ruin my woodworking fun.
--
Lee in Cave Junction, Oregon
Gateway to the Oregon Caves
Thanks, Lee ,Iv'e got over it but it took a couple of weeks .Wood working is my living and my hobby and it is still fun.Always a fresh challenge or technique to learn even customers that throw curve balls .I whine at the time but life would be boring without challenge!
To add a bit to what Stanley said, when the round bar stock is being machined it is important that there are no sharp corners. I can't remember all the details but leaving a small radius instead of a sharp edge on the part is critical if it is to be heat treated. A sharp edge creates a weak point which is almost always the point of failure. You might want to look at the router bit and see if there was a shoulder on the bit.
I had a similar experience with my first router and 1/4 in. (Craftsman). I didn't know that the bit was broken until I shut off the router. I think it was a combination of fatigue failure (vibration) and a cheap bit.
Just last week I broke 2 bits both 1/4" straight with 1/4" shank, a clean break between the end of the shank and the start of the cutter. When the first one broke I thought it was my fault so I bought the second one and paid close attention to my procedure, when it broke I returned them both(good quality money back guarantee) and bought the 1/2" shank.
My conclusion of the day was "such a good quality company should know better than that" Putting a 1/4" cutter on a 1/4" shank leaves a very small diameter (3/16?) of steel at the crossover point from shank to cutter. The heat affected zone caused by welding the cutters on and the stress points of the taper to the shoulder size all meet at this point. Add in some 30,000 rpm vibrations and its over very quickly.
This forum post is now archived. Commenting has been disabled